The present invention relates to coupling devices of the type used to transmit torque, for example, in a vehicle drive line, and more particularly, to such coupling devices of the type including a fluid pressure operated clutch assembly for controlling the transmission of torque through the coupling device.
As used herein, the term “coupling device” will be understood to mean and include a device which is able to transmit torque from an input to one or more outputs, and in which there is a clutch assembly disposed between the input and the output, such that the amount of torque transmitted is a function of the extent of engagement of the clutch assembly. Within the scope of the present invention, the term “coupling device” means and includes both gear-type devices (such as differentials), as well as gearless-type couplings.
Although the present invention may be utilized with many different types and configurations of coupling devices such as a coupling made in accordance with the teachings of U.S. Pat. No. 5,964,126 assigned to the assignee of the present invention and incorporated herein by reference, it is especially advantageous when utilized in conjunction with vehicle differentials of the general type illustrated and described in U.S. Pat. Nos. 5,310,388 and 6,464,056, both of which are also assigned to the assignee of the present invention and incorporated herein by reference.
In the differential coupling devices of the cited patents, there is a clutch pack operable to transmit torque between the input (housing connected to the ring gear) and the output (one of the axle shafts), with the degree of engagement of the clutch pack being determined by the fluid pressure in a clutch apply chamber. The fluid pressure biases a clutch engagement member (such as a piston) against the clutch pack. The differential coupling devices of the cited patents include a gerotor pump having one rotor fixed to rotate with the input, and the other rotor fixed to rotate with the output, such that the flow of pressurized fluid into the clutch apply chamber is generally proportional to the speed difference between the input and the output. As used herein, the term “clutch pack” will be understood to mean and include both a multiple friction disc type clutch pack, as well as any of the other well known types of clutch assemblies, such as cone clutches, in which the degree of engagement is generally proportional to the fluid pressure acting on the clutch piston, or on an equivalent clutch-engagement structure.
In differential coupling devices of the type described above, it is typical to provide a flow path from the clutch apply chamber to a reservoir or some other source of low pressure fluid, and to provide, as part of this “main” flow path, some sort of control valve which can control the flow from the clutch piston chamber to the low pressure source, thereby controlling the pressure in the clutch apply chamber, and therefore, controlling the “bias torque”, i.e., the extent to which torque is transmitted from the input to the output. It is also known, and within the scope of the present invention for the “main” flow path to communicate from a low pressure inlet (receiving fluid from a reservoir) to the inlet to the gerotor pump, and to have a control valve disposed in that flow path, thus controlling clutch apply pressure by limiting fluid flow into the inlet of the pump.
One of the problems associated with differential coupling devices of the type to which the present invention relates is that the housing of the coupling device typically rotates at approximately the speed of rotation of the axles, whereas the control valve and its associated structure, which must be operably associated with the coupling housing, must be stationary. Thus, there are references hereinafter to the “stationary control valve assembly”, as that term will be better understood subsequently, and which would typically, but not necessarily, include both a plenum member and a control valve. Therefore, at some location on the interface between the coupling housing and the control valve assembly, there is a need for a sealing arrangement which is capable of withstanding both the fairly high fluid pressure which is present in the high pressure side of the system, as well as the relatively high speeds of rotation of the coupling housing. By way of example only, in the commercial embodiment of the coupling device which has been developed by the assignee of the present invention, the fluid pressure in the high pressure portion of the system is typically in the range of about 500 psi to about 1000 psi, while the coupling housing may be rotating, relative to the stationary control valve assembly, at speeds in excess of 1500 rpm, corresponding to a vehicle speed of about 130 mph.
As has been well known to those skilled in the art of such coupling devices, the housing (“differential case”) has conventionally comprised one or more cast iron members, with any surfaces needing a particular flatness, smoothness, etc. being machined, while the rest of the member remains in the “as-cast” condition. In the commercial embodiment of the invention, as developed by the assignee of the present invention, the plenum and seal arrangement, which comprises part of the control valve assembly, is configured generally as in the above-incorporated U.S. Pat. No. 6,464,056, with a pair of high pressure seals received within the plenum and riding on the cylindrical surface of the housing hub portion. It has been observed, during the early stages of the development of the commercial embodiment, that the seal life of the high pressure seals was not nearly acceptable, but the reasons for the insufficient seal life were not at all apparent.
Although it was believed at the time that the surface of the cylindrical hub portion was satisfactory for engagement with the rotating high pressure seals, it has since been hypothesized that a condition which will be referred to hereinafter as “graphite porosity” was resulting in the cylindrical surface having small voids or porous locations, such that the surface, rather than being smooth, was actually serving to cut the sealing surface of the high pressure seals, as the hub portion rotated within the stationary seals. It is believed, based upon the observation of the parts tested during the development of the present invention, that the “graphite porosity” occurs because the seal being used includes a sealing element disposed in contact with the surface of the hub portion wherein the sealing element comprises a glass-fiber-reinforced member. As a result, when the seal and hub portion reach normal operating temperatures (in the range of about 200 to about 300 degrees F.), glass fibers from the sealing element project out of the element and are able to engage the graphite nodules as the hub portion rotates relative to the sealing element. Such engagement causes the graphite nodules to be pulled out of the surface of the hub portion, leaving pits in the surface (hence the term graphite “porosity”).
It has further been observed that the resulting pits or porosity on the surface of the hub portion subsequently collect pieces of the glass fibers, which thereafter may act as little “cutting tools” against the inside diameter of the sealing element, as the hub portion rotates within the seal assembly, and/or, it may be that the exposed edges of the pits act to cut the inside diameter of the sealing element. It is the above-described cutting action by either the glass fibers, or the edges of the pits, on the sealing element which is believed to be responsible for the observed, unsatisfactory sealing life, wherein the high pressure which the seals are able to maintain may rapidly decrease over a fairly short period of time.